fix(ci): use ubuntu-latest runner label instead of nonexistent docker label

act_runner's default registration only exposes ubuntu-latest (and
similar ubuntu-* labels); "docker" isn't a real label unless a runner
was explicitly configured with it.

.gitea/workflows/docker-publish.yml

@@ -11,7 +11,7 @@ permissions:
 
 jobs:
   build-and-push:
-    runs-on: docker
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4